1. Field of the Invention
This invention relates to new anisotropic compounds, and more particularly to new classes of cyclohexylcyclohexanoates and liquid crystal (LC) compositions containing these cyclohexylcyclohexanoates which can be used as dielectrics in liquid crystal devices. The invention also relates to a process for preparing the new anisotropic compounds and LC-mixtures containing the new anisotropic compounds.
2. Description of the Prior Art
Anisotropic biphenyl compounds of the formula (10) ##STR3## in which R' represents, for example, an alkyl group, have been disclosed, for example, in German Offenlegungsschrift No. 2,356,085, and are among the materials most often used for LC-mixtures.
It is also known from German Offenlegungsschrift No. 2,636,684 that the comparatively high viscosity and high .DELTA.n-values of the compounds of formula (10) can be decreased by replacing one of the phenylene rings by a trans-1,4-cyclohexylene ring. Such compounds are phenylcyclohexanes of the formula (20). ##STR4## in which R" has the same significance as R' in formula (10).
The .DELTA.n-values of anisotropic components of LC-mixtures are commonly used as a measure of the optical anisotropy and are calculated as the difference between the refractive indices parallel and perpendicular to the molecular axis of the particular material, that is, .DELTA.n=n.sub..parallel. -n.sub..perp..
For many types of LC-devices, especially the so-called "guest-host" effect cells, nematic LC-phases with the smallest possible .DELTA.n-values, i.e., not greater than 0.1, are required. By replacement of one of the phenylene rings in the compounds of formula (10) by a trans-cyclohexylene ring the .DELTA.n-value is decreased by nearly half (.DELTA.n.apprxeq.0.22 in compounds of formula (10). .DELTA.n.apprxeq.0.12 in compounds of formula (20)).
If the second phenylene ring of the anisotropic compounds of the formula (10), that is the remaining phenylene ring of the compounds of formula (20), is replaced by a cyclohexylene ring, the cyclohexylcyclohexanes of the formula (30) ##STR5## are obtained in which R'" has the same significance as R' in formula (10). These compounds ae disclosed in German Offenlegungsschrift No. 2,702,598. In these compounds the .DELTA.n-value is further decreased and, indeed, is less than 0.1, typically about 0.06.
However, the compounds of the formula (30) have the drawback (see R. Pohl et al., Phys. Letter 65A (2), 169(1978)) that the hydrogenation of the second phenylene ring produces a marked tendency toward the formation of smectic phases.
Therefore a need has continued to exist for LC-compounds having a low .DELTA.n-value combined with a low tendency for formation of smectic phases.